226 
THE SILURIAN VOLCANOES 
«00K IV 
out that while ordinary sedimeutatiou was in progress, an almost constant 
but variable discharge of fragmental materials t(wk place from the vents in 
the neigh bonrliood. Sometimes a special paroxysm of explosion would give 
rise to a distinct band of breccia or of tuff, but even where, during a time 
of comparative quiescence, tlie ordinaiy sand or mud predominated, it w'as 
generally mingled with more or less volcanic dust. 
Some bainls of conglomerate in this group of strata deserve particular 
notice. The most conspicuous of tliese, already referred to as seen at Porth 
Wen, is made up of quartz and quartzite blocks, embedded in a reddish matrix 
largelj' composed of ashy material, and recalling the red spotted tuffs of Llyn 
Padarn. Tlie occurrence, of strong conglomerates near tlie top of a volcanic 
series has Ijeen noted at 8t. David’s, Llyn Padarn and Bangor. In none of 
these localities, as I have tried to show, do the conglomerates mark an 
unconformability or serious break between two widely-separated groups of 
rock. The Anglesey section entirely supports this view, for the conglomerates 
are there merely intercalations in a continuous sequence of deposits ; they 
are succeeded by tuffs and shales like those which underlie them. The 
interposition of such coarse materials, however, may undoubtedly indicate 
local disturbance, connected, perhaps, in this and the other localities, 
with terrestrial readjustments consequent upon the waning of volcanic 
energy. 
The detailed geological structure of Anglesey is still far from being com- 
pletely understood. Besides the seri(jxis crushing here referred to, there is 
reason to suspect that considerable plication, perhaps even inversion, of the 
strata has taken place, and that, by denudation, detached portions of some 
of the higher groups have been left in different parts of the island. The 
occurrence of U])per Silurian fossils in several localities adds to the per- 
plexity of the problem liy iiulicatiug that, among the folds and hardly dis- 
tinguishable from the older slates, portions of Upper Silurian formations 
may have been caught and presers e,d. These difficulties, moreover, involve 
in some obscurity the closing phases of volcanic activity in Wales ; for until 
they are, to some extent at least, removed, we shall be left in doubt whether 
the vents in the north of Anglesey, which were in eruption probably during 
Bala time, were the last of the long succession of Welsh volcanoes. If the 
black shales of Fary.s Mountain are really referable to the horizon of the 
Majdiill Sandstone, the two great igneous bands between which they lie would 
seem to mark an outbreak of volcarric energy during Upper Silurian time. 
Xo other indication.s, however, of eruptions of that age having been met 
with in Great Britain (though they occur in the south-west of Ireland and 
possibly in Gloucestershire), more careful investigation is required before 
such a position can Ire safely assigned to any rocks in Anglesey. 
Butting these dou))tful rocks aside for the present, we may, in conclusion, 
contrast tlie type of eruption in Anglesey with that of the great Snowdonian 
region. While the Caernarvonshire volcanoes were pouring forth their 
\'olumes of felsitic lava, and piling them up for thousands of feet on the 
sea-floor, the northern Anglesey Agents, not more than some tive-and-tweuty 
